A dental composite restorative material is a typical dental curable composition. In a dental clinic, for example, the dental composite restorative material is filled into a cavity of a tooth to be restored, and then formed into the tooth shape. Subsequently, the formed dental composite restorative material is polymerized and cured by irradiation with active light with the use of a special irradiator. In this manner, the damaged tooth is restored.
In a dental laboratory, the dental composite restorative material is built-up in the shape of a tooth to be restored on a plaster cast, and then polymerized and cured by irradiation with light. The resulting prosthesis is, in a dental clinic, bonded to the tooth with a dental adhesive. In this manner, the damaged tooth is restored.
A dental composite restorative material is advantageous in that, this can provide substantially the same color as that of a natural tooth, and has good handling property. In recent years, therefore, the dental composite restorative material has spread rapidly, and now is applied in most of the treatments for anterior teeth. Furthermore, a dental composite restorative material has also been developed which provides the cured product thereof with considerably high mechanical strength. As a result, the dental composite restorative material is beginning to be applied also in the restoration of a posterior tooth, to which high bite pressure is loaded.
A dental composite restorative material generally includes a polymerizable monomer (monomer), a filler, and a polymerization initiator as main components. Furthermore, when the filler is added to the dental composite restorative material, the type, shape, particle diameter, loading amount and the like of the filler to be used are chosen. When they are chosen appropriately, various properties such as the handling property of the paste-like dental composite restorative material, and the esthetics and mechanical strength of the cured product are controlled optimally.
For example, when inorganic fillers with a large particle diameter are added to a dental composite restorative material, the resulting composite restorative material can form a cured product with high mechanical strength. Unfortunately, however, the cured product may have reduced surface smoothness or wear resistance. As a result, it is difficult to obtain a cured product with a glossy finish surface like a natural tooth.
Furthermore, adding fine inorganic fillers with an average particle diameter of 1 μm or less into a dental composite restorative material can provide a cured product with good surface smoothness or wear resistance. However, such fine inorganic fillers have a large specific surface area, thereby significantly increasing the viscosity of a paste-like composite restorative material. In the treatment of a tooth, it is necessary for a dentist to previously adjust the viscosity of a dental composite restorative material to a level suitable for use in the oral cavity. In order to reduce the viscosity, it is necessary to reduce the content of fine inorganic fillers. However, such a reduction of the content of fine inorganic fillers may result in an increase in the shrinkage of the resulting cured product, which is associated with the polymerization of a monomer during the curing of the composite restorative material, a reduction in the mechanical strength of the resulting cured product, and the like.
Under such circumstances, the use of an organic-inorganic composite filler is proposed (see for example Patent Literatures 1 and 2). According to these Patent Literatures, using such an organic-inorganic composite filler may provide a paste-like composite restorative material having good handling property, while maintaining good surface smoothness and wear resistance of the cured product as in a case where a fine inorganic filler is used, and further reduces the polymerization shrinkage of the cured product.
Such an organic-inorganic composite filler is a composite filler including an organic resin in which fine inorganic fillers are contained. The organic-inorganic composite fillers have a surface area smaller than that of the above mentioned fine inorganic fillers. Therefore, even adding a sufficient amount of the organic-inorganic composite filler can obtain a paste-like composite restorative material without causing thickening.
A general method for producing the above mentioned organic-inorganic composite filler includes previously kneading fine inorganic fillers and a polymerizable monomer to form a curable composition, polymerizing and curing the curable composition to form, a cured product, and then grinding the cured product (see paragraph [0012] of Patent Literature 1).
There is also known a method for producing an organic-inorganic composite filler with a narrow particle size distribution (see Claims of Patent Literature 2). This method includes firstly granulating fine inorganic fillers by a method such as spray drying to produce inorganic aggregate particles. Subsequently, the method includes bringing the produced inorganic-aggregate particles into contact with a liquid polymerizable monomer under reduced pressure, and then returning the reduced pressure to the normal pressure. According to this step, the polymerizable monomer is allowed to penetrate aggregation gaps among primary particles constituting the inorganic aggregate particles. Subsequently, the method includes polymerizing and curing the penetrating monomer to form an organic-inorganic composite filler. This organic-inorganic composite filler may also be used without being ground.
The Literature describes that in the producing method, the polymerizable monomer may be diluted with a volatile solvent when the inorganic aggregate particles are brought into contact with the polymerizable monomer (paragraphs [0042] to [0043]). The reason is that the polymerizable monomer should be allowed to sufficiently penetrate the aggregation, gaps among the inorganic aggregate particles.
Unfortunately, the description in the specification of the Literature is silent on how the volatile solvent should be used. The description is also silent on what process should be specifically used to remove the volatile solvent after the monomer penetrates the aggregation gaps. Furthermore, the Literature discloses a dropping or continuous mixing method as a method for bringing the polymerizable monomer into contact with the inorganic aggregate particles. Such a continuous contact method is directed to allow as much the polymerizable monomer as possible to penetrate the aggregation gaps among the inorganic aggregate particles.
From the above description, therefore, it is apparent that also in an aspect of diluting the above mentioned polymerizable monomer with a volatile solvent, the technical idea is unchanged of loading the polymerizable monomer as much as possible into the aggregation gaps.
In other words, it suggests that the volatile solvent should be used in a necessary minimum amount. Therefore, it is considered that in an aspect of diluting the polymerizable monomer with a volatile solvent, the diluting solution of the polymerizable monomer is allowed to penetrate the aggregation gaps, while the volatile solvent is evaporated from the penetrating diluting solution in parallel.
Thus, it is considered chat the dilution aspect is intended to fill the whole aggregation gap with a sufficient amount of the polymerizable monomer through the continuous penetration and evaporation before the polymerization and curing.